i used to drive sewage tankers and have emptied hundreds of tanks .best i can say is dont use any chemicals in your toilet .use a tea spoon of vinegar if required or a tablet of cheap washing powder. sewage breaks down much quicker if we use natural bacteria . never wash the tank part of your toilet . inside . you can the outside . just tip and use again. let the bacteria grow. even better leave a smidgen in the bottom of cassette . if you do go on sites you will possibly have seen special drop off points for chemical toilets thats because chemicals from pota potties kills the sewage works on site. its an incredible cost for them to empty the works and import good bacteria ridden s--t. even south west water sites here get into trouble in summer with holiday makers dumping too much chemical contaminated s--t into toilets in village toilets. sewage is a very interesting science. i also worked fresh water sites and that is even more interesting. all campers should visit a local water /site and get info.